1. Field of the Invention
This invention relates to polyimides having improved thermal-oxidative stability and to the process of preparing said polyimides derived from the polymerization of effective amounts of polyamines, tetracarboxylic dianhydrides and novel dicarboxylic endcaps. More specifically, this invention relates to all types of PMR (polymerization monomeric reactants) polyimides and to the use of these novel polyimides in the preparation of PMR resins and composites. The polyimides are derived from the polymerization of various polyamines, e.g. aromatic diamines, tetracarboxylic dianhydrides and a novel class of dicarboxylic endcaps. These dicarboxylic endcaps were found to substantially increase the lifetime and use temperatures of polyimide matrix composites allowing their use in various engine parts such as inlets, fan ducts, exit flaps etc. for high speed aircraft and the like. Ultimately, the payoff for using these novel polyimides in developing polymer composites that can operate at higher temperatures is the weight savings of 10 to 20% over other engine parts made from conventional materials.
2. Description of Related Prior Art
In structural applications, fiber-reinforced high-temperature polyimide matrix composites offers significant advantages over other materials because of their low density and high specific strength. These composites are attractive for use in aerospace systems, e.g. aircraft engines, airframe, missiles, and rockets, where weight is critical. This weight reduction has substantial benefits in terms of fuel savings, an increased passenger, cargo load, or increased speed and maneuverability. The durability and reliability of materials used in aerospace components is a critical concern. Among the materials requirements for these applications are a high glass-transition temperature, T.sub.g, (at least 25.degree. C. higher than the intended use temperature), good high-temperature stability in a variety of environments, and good mechanical properties over a wide range of temperatures. In general, the stability and T.sub.g of most organic polymers limit their use, at best, to applications in which temperatures are not higher than 350 to 370.degree. l C.
Addition-curing polyimides were investigated in an attempt to improve the processability of condensation polyimides without adversely affecting their stability and high-temperature performance. The most noteworthy development is the Polymerization of Monomer Reactant (PMR) family of polyimides, in particular PMR-15, developed at the NASA Lewis Research Center. For PMR-15, reinforcement fibers are impregnated with a solution of the dialkyl ester of 3,3",4,4'-benzophenone tetracarboxylic acid (BTDE), methylene dianiline (MDA), and the monoalkyl ester of 5-norbornene-2,3-dicarboxylic acid (NE), in a low boiling solvent, typically methanol or ethanol. In the first step of this process, these monomers undergo imidization at temperatures around 200.degree. C. to yield short-chain norbornene end-capped polyimide oligomers, (Mw.sub.theoretical =1500 for PMR-15). At temperatures above 300.degree. C., these oligomers undergo a cross-linking reaction involving the norbornene endcap.
In the PMR system, melt viscosity and resin flow can be controlled, to some extend, by varying monomer stoichiometry, which enables the removal of volatile by-products produced in the condensation reaction and leads to composites with fairly low-void contents. PMR-15 has a T.sub.g after post-cure of 365.degree. C. and has good retention of mechanical properties and low weight losses in air for long exposure times (&gt;10,000 h) at temperatures up to 230.degree. C. and for shorter times at temperatures as high as 316.degree. C. While norbornene end-caps have been used successfully in a number of polyimide systems, there are some drawbacks to their use. The thermal-oxidative stability of the norbornene ring is poor due to the large amount of saturated carbons present in this structure. For this reason, the norbornene end-cap and the structures that result from its cross-linking often become the oxidative weak link in polyimides in which it is used. In addition, processing problems can be encountered with norbornene end-capped polymers due to the potential for formation of cyclopentadiene during cross-linking. These deficiencies have prompted the search for new addition-curable end-caps for polyimides.
For example, Jones and co-workers (Jou. Of Polymer Science, Chem. Ed. 33:767-70.) utilized the Diels-Alder cycloaddition between bis(furyl)imide and BMI to produce polyimide. The authors postulate that dehydration occurs between 204 and 288.degree. C., thus leading to a more stable polyimide. Carbon fiber-reinforced composites processed at 400.degree. F., and post-cured at 600.degree. F. had good initial flexural strength at room temperature (123.5 Ksi) and at 600.degree. F. (127.5 Ksi). These composites had modest thermal-oxidative stability; samples aged in air for 1000 hours at 316.degree. C. had an 18% weight loss. Flexural strength of these composites dropped from 172 to 66 Ksi after aging under these conditions.
In the prior art, U.S. Pat. No. 4,739,030 (H. R. Lubowitz et al.) discloses difunctional, crosslinking end-cap monomers useful in the synthesis of high performance oligomers that contain polysulfone or polyethersulfone backbones. The difunctional end-cap monomer useful in the preparation of oligomers are resistant to organic solvents comprise various imidophenols that contain the norbornene group. U.S. Pat. No. 5,594,089 (H. R Lubowitz et al.) discloses linear or multidimensional heterocycle or heterocycle sulfone oligomer having two or four crosslinking sites at each end of its backbone. The multiple chemically functional end cap monomers include organic compounds containing the norbornene group. U.S. Pat. Nos. 5,756,597; 5,817,744; and 5,969,079 (H. R. Lubowitz et al.) further disclose resins that are improved by forming four crosslinks at each addition polymerization site in the backbone of the resin using crosslinking functionalities of a general formula which contain the norbornene group.
U.S. Pat. No. 5,145,943 (J. Y. Lee et al.) discloses polyimides derived from 4,4'-methylenedianiline (MDA), 3,3',4,4'-benzophenonetetracarboxylic acid methyl ester (BTDE), and 5-norbornene-2,3-dicarboxylic acid methyl ester (NDE) with molar ratio of 3:2:2. U.S. Pat. Nos. 3,745,149 and 5,338,827 (Serafini et al.) disclose polyimides prepared by the reaction of a mixture of monomers comprising dialkyl ester of an aromatic tetracarboxylic acid, an aromatic diamine, and a monoalkyl or dialkyl ester of a dicarboxylic acid where the ratio is n:(n+1):2, wherein n has a value of from 1 to 20. Patentees disclosed polyimides that can be processed from a mixture of monomeric reactants using lower alcohols to esterify an anhydride endcap and an aromatic dianhydride. These monomeric reactants when combined with an aromatic diamine form a monomeric mixture which at high temperature polymerize to a polyimide. This procedure was the evolution of the terminology PMR (polymerization of monomeric reactants).